A color image is formed in a color photographic element by generation of an imagewise distribution of the dye as a function of silver halide development. Typically dye is formed by reaction between a coupler compound and oxidized silver halide developing agent. However, dye images can also be formed by other means such as the imagewise diffusion of dye or the imagewise destruction of dye. Regardless of the means by which the dye image is formed, it is common practice to remove the silver image formed on development and silver halide remaining in unexposed areas of the element. This is accomplished by a bleaching step in which the silver image is oxidized by a suitable oxidizing agent, commonly referred to as a bleaching agent, following by dissolving the silver halide in a silver halide solvent, commonly referred to as a fixing agent. Alternatively, the bleaching agent and the fixing agent can be combined in a bleach-fixing solution and the silver removed in one step by the use of such a solution.
A variety of bleach compositions have been employed with silver halide materials. These include those based on, i.e., ferricyanide, dichromate, permanganate, chloride, quinone, as well as amino carboxylic acid and persulfate oxidizing agents. Typical compositions containing such oxidizing agents for the bleaching of photographic image silver are referred to or described in U.S. Pat. Nos. 3,512,979; 3,615,513; 3,647,469; 3,689,272; 3,706,561; 3,716,362; 3,749,572; 3,772,020; 3,893,858; 4,163,669; 4,292,401; 4,293,639; 4,301,236; 4,322,493; 4,448,878; 4,458,010; 4,481,290; 4,524,129; 4,546,070; and 4,596,764.
Other descriptions of the composition and use of photographic silver bleaches are found on pages 124 and 125 of Neblettes's Handbook of PHOTOGRAPHY AND REPROGRAPHY, Materials, Processes and Systems, Seventh Edition, Van Nostrand Reinhold Company; and on pages 447 to 450 of THE THEORY OF THE PHOTOGRAPHIC PROCESS, Fourth Edition, MacMillan Publishing Co., Inc., Hew York, London.
Various substituted thiols have been found to accelerate bleaching either when directly added to the bleach solution, or, if introduced via a prebath, before the bleaching step. See for example U.S. Pat. No. 3,893,858, issued July 8, 1975.
U.S. Pat. No. 4,163,669, issued Aug. 7, 1979, suggest that there would be an advantege to incorporating the bleach accelerator directly in the photographic film but that incorporation of the free thiol would have adverse effects on the photographic properties of the film. The '669 patent suggests incorporating in a photographic element certain bleach accelerators as the salt of a heavy metal ion. Among the bleach accelerators mentioned are the organic thiols of the type described in U.S. Patent 3,893,858.
While the heavy metal salts described in the '669 patent are effective with bleach compositions based on ferric EDTA, they are not as effective with other bleach compositions, such as persulfate bleaches, as would be desired. Furthermore, while they give good sensitometric results with freshly coated photographic elements, upon prolonged keeping they have a deleterious effect on the sensitometric properties of the material.
Accordingly it would be desirable to have bleach accelerator precursor compounds which can be incorporated in a photographic film without adversely affecting the sensitometric properties of the film upon long term keeping and which will permit the film to be used in a multiplicity of bleach and bleach fix compositions with good results.